This invention relates to coal liquefaction processing. Coal liquefaction is of course known. Direct liquefaction is a process in which coal is converted to liquid products. This is accomplished by adding hydrogen to coal that has been slurried in a solvent, with the hydrogen addition taking place at elevated temperatures and pressures. The process was invented in 1913 and has remained relatively unchanged since its extensive use by Germany during World War II. Currently, direct liquefaction processes are usually performed in two stages using high-rank coals and operated at temperatures greater than 400.degree. C. and pressures of approximately 2500 psi. Catalysts, which are very expensive, are generally used in both stages. While the economics of direct liquefaction have improved during the last five years, it is not yet a commercially viable method of producing liquid fuels due to its use of catalysts in both stages and the severity of the conditions employed.
Work performed early in the 1980's showed that low-rank coals are very reactive. It seems logical that they could be cost-effectively substituted into the direct liquefaction process since they would most likely require less severe reaction conditions. However, most attempts to use low-rank coals during direct liquefaction have not successfully produced high yields of top-quality liquid product possibly because the coal reacts too rapidly for the available hydrogen source(s), resulting in the production of retrograded material. Any successful use of low-rank coals in direct liquefaction requires that advantage be taken of their positive characteristics. This is highly unlikely with the current direct liquefaction approach, which assumes that the reactions occurring at the usually severe conditions provide the best (or only) way to convert coal to liquid products.
In short, currently-used liquefaction processes do not provide high yields of desirable liquid products for low-rank coals because it has generally been assumed in the past that the conditions suitable for high-rank coal would also be suitable for low-rank coal. This has been found to not be the case because the coals are inherently different.
Low-rank coals have higher ash and moisture contents than high-rank coals. More importantly, low-rank coals contain more oxygen (on a moisture-free basis) than do high-rank coals. It is their high oxygen content which makes them quite reactive. A typical low-rank coal would have a moisture content within the range 25 wt. % to 67 wt. %, an ash content of &lt;1 wt. % to 40 wt. %, and an oxygen content within the range of from 15 wt. % to 25 wt. %, all values on a moisture-free basis.
Typical coals that can be classified as low-rank include brown coal, lignite, and subbituminous coal. The present invention may make use of virtually any organic carbonaceous material as a feedstock, including, but not limited to, low-rank coals, high-rank coals, leonardite, peat, wood, biomass, and even organic garbage. The yield and quality of the product would be determined by the choice of organic feedstock. However, the present invention provides the opportunity to attain relatively high conversions to liquid products for low-rank coals and poor-quality organic materials.
The present invention was conceived by viewing direct coal liquefaction in a manner that is substantially different than is currently typical. For this invention, the initial treatment stage is regarded as the initial solubilization of the coal in a solvent such that it is, in effect, reversed coalification. Subsequent upgrading can then be thought of as refining. The underlying assumption of this type of processing, demonstrated by the higher yields of desirable liquid products achieved by this invention using low-rank coal, is that the structure of the coal is comprised of physically and chemically tangled, highly cross-linked molecules. The molecular structures of petroleum distillate fuels (the desired product) by comparison, are discreet molecules of similar size and chemical nature, having virtually no chemical or physical attachments. The goal of the process is therefore to go from a chemical "knot" to an orderly structure. In this process, the first phase liquefaction should therefore be to "untangle" the coal structure at low-severity conditions to prevent coalification (retrograde) reactions, while the second phase should be to "organize" the untangled pieces so that those of similar size and chemical nature are first separated from the remaining material and then stabilized to prevent back reactions. In accordance with this mechanistic view, the present invention was specifically designed for low-rank coals to accomplish high-yield liquefaction with minimum retrograde reactions.
The purpose of liquefaction is to provide economical sources of carbonaceous liquid product which may be used either as energy sources or in creating other products. If the economics are incorrect, i.e., if the yield of desired liquid product is low, then the whole idea of using the process is simply defeated. Importantly too, is the fact that because large volumes of materials are treated, very small differences in efficiency or liquid product yield mean substantially significant differences in process economics.
Accordingly, it can be seen that there is a continuing need for the development of coal liquefaction processes which are specifically tailored to low-rank coals and other organic carbonaceous materials, and which provide high yields of desired liquid products. This invention has as its primary objective the fulfillment of this need.
The method and manner of accomplishing this objective will become apparent from the detailed description which follow hereinafter.